What to give the person who has everything? A clock that will keep perfect time  even after the heat-death of the universe. 

Such a “space-time crystal” has periodic structure in time as well as space. Why haven’t we built those? With such a 4D crystal, scientists would have a new and more effective means by which to study how complex physical properties and behaviors emerge from the collective interactions of large numbers of individual particles, the so-called many-body problem of physics. A space-time crystal could also be used to study phenomena in the quantum world, such as entanglement, in which an action on one particle impacts another particle even if the two particles are separated by vast distances.


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